/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
 *
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 *
 * Written by Doug Lea with assistance from members of JCP JSR-166 Expert Group and released to the public domain, as
 * explained at http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import java.util.concurrent.TimeUnit;

import sun.misc.Unsafe;

/**
 * Provides a framework for implementing blocking locks and related synchronizers (semaphores, events, etc) that rely on
 * first-in-first-out (FIFO) wait queues. This class is designed to be a useful basis for most kinds of synchronizers
 * that rely on a single atomic {@code int} value to represent state. Subclasses must define the protected methods that
 * change this state, and which define what that state means in terms of this object being acquired or released. Given
 * these, the other methods in this class carry out all queuing and blocking mechanics. Subclasses can maintain other
 * state fields, but only the atomically updated {@code int} value manipulated using methods {@link #getState},
 * {@link #setState} and {@link #compareAndSetState} is tracked with respect to synchronization.
 *
 * <p>
 * Subclasses should be defined as non-public internal helper classes that are used to implement the synchronization
 * properties of their enclosing class. Class {@code AbstractQueuedSynchronizer} does not implement any synchronization
 * interface. Instead it defines methods such as {@link #acquireInterruptibly} that can be invoked as appropriate by
 * concrete locks and related synchronizers to implement their public methods.
 *
 * <p>
 * This class supports either or both a default <em>exclusive</em> mode and a <em>shared</em> mode. When acquired in
 * exclusive mode, attempted acquires by other threads cannot succeed. Shared mode acquires by multiple threads may (but
 * need not) succeed. This class does not &quot;understand&quot; these differences except in the mechanical sense that
 * when a shared mode acquire succeeds, the next waiting thread (if one exists) must also determine whether it can
 * acquire as well. Threads waiting in the different modes share the same FIFO queue. Usually, implementation subclasses
 * support only one of these modes, but both can come into play for example in a {@link ReadWriteLock}. Subclasses that
 * support only exclusive or only shared modes need not define the methods supporting the unused mode.
 *
 * <p>
 * This class defines a nested {@link ConditionObject} class that can be used as a {@link Condition} implementation by
 * subclasses supporting exclusive mode for which method {@link #isHeldExclusively} reports whether synchronization is
 * exclusively held with respect to the current thread, method {@link #release} invoked with the current
 * {@link #getState} value fully releases this object, and {@link #acquire}, given this saved state value, eventually
 * restores this object to its previous acquired state. No {@code AbstractQueuedSynchronizer} method otherwise creates
 * such a condition, so if this constraint cannot be met, do not use it. The behavior of {@link ConditionObject} depends
 * of course on the semantics of its synchronizer implementation.
 *
 * <p>
 * This class provides inspection, instrumentation, and monitoring methods for the internal queue, as well as similar
 * methods for condition objects. These can be exported as desired into classes using an
 * {@code AbstractQueuedSynchronizer} for their synchronization mechanics.
 *
 * <p>
 * Serialization of this class stores only the underlying atomic integer maintaining state, so deserialized objects have
 * empty thread queues. Typical subclasses requiring serializability will define a {@code readObject} method that
 * restores this to a known initial state upon deserialization.
 *
 * <h3>Usage</h3>
 *
 * <p>
 * To use this class as the basis of a synchronizer, redefine the following methods, as applicable, by inspecting and/or
 * modifying the synchronization state using {@link #getState}, {@link #setState} and/or {@link #compareAndSetState}:
 *
 * <ul>
 * <li>{@link #tryAcquire}
 * <li>{@link #tryRelease}
 * <li>{@link #tryAcquireShared}
 * <li>{@link #tryReleaseShared}
 * <li>{@link #isHeldExclusively}
 * </ul>
 *
 * Each of these methods by default throws {@link UnsupportedOperationException}. Implementations of these methods must
 * be internally thread-safe, and should in general be short and not block. Defining these methods is the <em>only</em>
 * supported means of using this class. All other methods are declared {@code final} because they cannot be
 * independently varied.
 *
 * <p>
 * You may also find the inherited methods from {@link AbstractOwnableSynchronizer} useful to keep track of the thread
 * owning an exclusive synchronizer. You are encouraged to use them -- this enables monitoring and diagnostic tools to
 * assist users in determining which threads hold locks.
 *
 * <p>
 * Even though this class is based on an internal FIFO queue, it does not automatically enforce FIFO acquisition
 * policies. The core of exclusive synchronization takes the form:
 *
 * <pre>
 * Acquire:
 *     while (!tryAcquire(arg)) {
 *        <em>enqueue thread if it is not already queued</em>;
 *        <em>possibly block current thread</em>;
 *     }
 *
 * Release:
 *     if (tryRelease(arg))
 *        <em>unblock the first queued thread</em>;
 * </pre>
 *
 * (Shared mode is similar but may involve cascading signals.)
 *
 * <p id="barging">
 * Because checks in acquire are invoked before enqueuing, a newly acquiring thread may <em>barge</em> ahead of others
 * that are blocked and queued. However, you can, if desired, define {@code tryAcquire} and/or {@code tryAcquireShared}
 * to disable barging by internally invoking one or more of the inspection methods, thereby providing a <em>fair</em>
 * FIFO acquisition order. In particular, most fair synchronizers can define {@code tryAcquire} to return {@code false}
 * if {@link #hasQueuedPredecessors} (a method specifically designed to be used by fair synchronizers) returns
 * {@code true}. Other variations are possible.
 *
 * <p>
 * Throughput and scalability are generally highest for the default barging (also known as <em>greedy</em>,
 * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy. While this is not guaranteed to be fair or
 * starvation-free, earlier queued threads are allowed to recontend before later queued threads, and each recontention
 * has an unbiased chance to succeed against incoming threads. Also, while acquires do not &quot;spin&quot; in the usual
 * sense, they may perform multiple invocations of {@code tryAcquire} interspersed with other computations before
 * blocking. This gives most of the benefits of spins when exclusive synchronization is only briefly held, without most
 * of the liabilities when it isn't. If so desired, you can augment this by preceding calls to acquire methods with
 * "fast-path" checks, possibly prechecking {@link #hasContended} and/or {@link #hasQueuedThreads} to only do so if the
 * synchronizer is likely not to be contended.
 *
 * <p>
 * This class provides an efficient and scalable basis for synchronization in part by specializing its range of use to
 * synchronizers that can rely on {@code int} state, acquire, and release parameters, and an internal FIFO wait queue.
 * When this does not suffice, you can build synchronizers from a lower level using {@link java.util.concurrent.atomic
 * atomic} classes, your own custom {@link java.util.Queue} classes, and {@link LockSupport} blocking support.
 *
 * <h3>Usage Examples</h3>
 *
 * <p>
 * Here is a non-reentrant mutual exclusion lock class that uses the value zero to represent the unlocked state, and one
 * to represent the locked state. While a non-reentrant lock does not strictly require recording of the current owner
 * thread, this class does so anyway to make usage easier to monitor. It also supports conditions and exposes one of the
 * instrumentation methods:
 *
 * <pre>
 * {
 *     &#64;code
 *     class Mutex implements Lock, java.io.Serializable {
 *
 *         // Our internal helper class
 *         private static class Sync extends AbstractQueuedSynchronizer {
 *             // Reports whether in locked state
 *             protected boolean isHeldExclusively() {
 *                 return getState() == 1;
 *             }
 *
 *             // Acquires the lock if state is zero
 *             public boolean tryAcquire(int acquires) {
 *                 assert acquires == 1; // Otherwise unused
 *                 if (compareAndSetState(0, 1)) {
 *                     setExclusiveOwnerThread(Thread.currentThread());
 *                     return true;
 *                 }
 *                 return false;
 *             }
 *
 *             // Releases the lock by setting state to zero
 *             protected boolean tryRelease(int releases) {
 *                 assert releases == 1; // Otherwise unused
 *                 if (getState() == 0)
 *                     throw new IllegalMonitorStateException();
 *                 setExclusiveOwnerThread(null);
 *                 setState(0);
 *                 return true;
 *             }
 *
 *             // Provides a Condition
 *             Condition newCondition() {
 *                 return new ConditionObject();
 *             }
 *
 *             // Deserializes properly
 *             private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
 *                 s.defaultReadObject();
 *                 setState(0); // reset to unlocked state
 *             }
 *         }
 *
 *         // The sync object does all the hard work. We just forward to it.
 *         private final Sync sync = new Sync();
 *
 *         public void lock() {
 *             sync.acquire(1);
 *         }
 * 
 *         public boolean tryLock() {
 *             return sync.tryAcquire(1);
 *         }
 * 
 *         public void unlock() {
 *             sync.release(1);
 *         }
 * 
 *         public Condition newCondition() {
 *             return sync.newCondition();
 *         }
 * 
 *         public boolean isLocked() {
 *             return sync.isHeldExclusively();
 *         }
 * 
 *         public boolean hasQueuedThreads() {
 *             return sync.hasQueuedThreads();
 *         }
 * 
 *         public void lockInterruptibly() throws InterruptedException {
 *             sync.acquireInterruptibly(1);
 *         }
 * 
 *         public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
 *             return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 *         }
 *     }
 * }
 * </pre>
 *
 * <p>
 * Here is a latch class that is like a {@link java.util.concurrent.CountDownLatch CountDownLatch} except that it only
 * requires a single {@code signal} to fire. Because a latch is non-exclusive, it uses the {@code shared} acquire and
 * release methods.
 *
 * <pre>
 * {
 *     &#64;code
 *     class BooleanLatch {
 *
 *         private static class Sync extends AbstractQueuedSynchronizer {
 *             boolean isSignalled() {
 *                 return getState() != 0;
 *             }
 *
 *             protected int tryAcquireShared(int ignore) {
 *                 return isSignalled() ? 1 : -1;
 *             }
 *
 *             protected boolean tryReleaseShared(int ignore) {
 *                 setState(1);
 *                 return true;
 *             }
 *         }
 *
 *         private final Sync sync = new Sync();
 * 
 *         public boolean isSignalled() {
 *             return sync.isSignalled();
 *         }
 * 
 *         public void signal() {
 *             sync.releaseShared(1);
 *         }
 * 
 *         public void await() throws InterruptedException {
 *             sync.acquireSharedInterruptibly(1);
 *         }
 *     }
 * }
 * </pre>
 *
 * @since 1.5
 * @author Doug Lea
 */
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable {

    private static final long serialVersionUID = 7373984972572414691L;

    /**
     * Creates a new {@code AbstractQueuedSynchronizer} instance with initial synchronization state of zero.
     */
    protected AbstractQueuedSynchronizer() {}

    /**
     * Wait queue node class.
     *
     * <p>
     * The wait queue is a variant of a "CLH" (Craig, Landin, and Hagersten) lock queue. CLH locks are normally used for
     * spinlocks. We instead use them for blocking synchronizers, but use the same basic tactic of holding some of the
     * control information about a thread in the predecessor of its node. A "status" field in each node keeps track of
     * whether a thread should block. A node is signalled when its predecessor releases. Each node of the queue
     * otherwise serves as a specific-notification-style monitor holding a single waiting thread. The status field does
     * NOT control whether threads are granted locks etc though. A thread may try to acquire if it is first in the
     * queue. But being first does not guarantee success; it only gives the right to contend. So the currently released
     * contender thread may need to rewait.
     *
     * <p>
     * To enqueue into a CLH lock, you atomically splice it in as new tail. To dequeue, you just set the head field.
     * 
     * <pre>
     *      +------+  prev +-----+       +-----+
     * head |      | <---- |     | <---- |     |  tail
     *      +------+       +-----+       +-----+
     * </pre>
     *
     * <p>
     * Insertion into a CLH queue requires only a single atomic operation on "tail", so there is a simple atomic point
     * of demarcation from unqueued to queued. Similarly, dequeuing involves only updating the "head". However, it takes
     * a bit more work for nodes to determine who their successors are, in part to deal with possible cancellation due
     * to timeouts and interrupts.
     *
     * <p>
     * The "prev" links (not used in original CLH locks), are mainly needed to handle cancellation. If a node is
     * cancelled, its successor is (normally) relinked to a non-cancelled predecessor. For explanation of similar
     * mechanics in the case of spin locks, see the papers by Scott and Scherer at
     * http://www.cs.rochester.edu/u/scott/synchronization/
     *
     * <p>
     * We also use "next" links to implement blocking mechanics. The thread id for each node is kept in its own node, so
     * a predecessor signals the next node to wake up by traversing next link to determine which thread it is.
     * Determination of successor must avoid races with newly queued nodes to set the "next" fields of their
     * predecessors. This is solved when necessary by checking backwards from the atomically updated "tail" when a
     * node's successor appears to be null. (Or, said differently, the next-links are an optimization so that we don't
     * usually need a backward scan.)
     *
     * <p>
     * Cancellation introduces some conservatism to the basic algorithms. Since we must poll for cancellation of other
     * nodes, we can miss noticing whether a cancelled node is ahead or behind us. This is dealt with by always
     * unparking successors upon cancellation, allowing them to stabilize on a new predecessor, unless we can identify
     * an uncancelled predecessor who will carry this responsibility.
     *
     * <p>
     * CLH queues need a dummy header node to get started. But we don't create them on construction, because it would be
     * wasted effort if there is never contention. Instead, the node is constructed and head and tail pointers are set
     * upon first contention.
     *
     * <p>
     * Threads waiting on Conditions use the same nodes, but use an additional link. Conditions only need to link nodes
     * in simple (non-concurrent) linked queues because they are only accessed when exclusively held. Upon await, a node
     * is inserted into a condition queue. Upon signal, the node is transferred to the main queue. A special value of
     * status field is used to mark which queue a node is on.
     *
     * <p>
     * Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill Scherer and Michael Scott, along with members of
     * JSR-166 expert group, for helpful ideas, discussions, and critiques on the design of this class.
     *
     * Java中的CLH 队列是原CLH队列的一个变种,线程由原自旋机制改为阻塞机制。
     */
    static final class Node {
        /** Marker to indicate a node is waiting in shared mode */
        static final Node SHARED = new Node();
        /** Marker to indicate a node is waiting in exclusive mode */
        static final Node EXCLUSIVE = null;

        // 在同步队列中等待的线程等待超时或者被中断，需要从同步队列中取消等待
        /** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED = 1;

        // 后继节点的线程处于等待状态，而当前的节点如果释放了同步状态或者被取消，将会通知后继节点，使后继节点的线程得以运行
        /** waitStatus value to indicate successor's thread needs unparking */
        static final int SIGNAL = -1;

        // 节点在等待队列中，节点的线程等待在condition上，当其他线程对condition调用了signal（）方法后，该节点会从等待队列中转移到同步队列中，加入到同步队列的获取中
        /** waitStatus value to indicate thread is waiting on condition */
        static final int CONDITION = -2;

        // 表示下一次共享式同步状态获取将会被无条件地传播下去
        /**
         * waitStatus value to indicate the next acquireShared should unconditionally propagate
         */
        static final int PROPAGATE = -3;

        /**
         * Status field, taking on only the values: SIGNAL: The successor of this node is (or will soon be) blocked (via
         * park), so the current node must unpark its successor when it releases or cancels. To avoid races, acquire
         * methods must first indicate they need a signal, then retry the atomic acquire, and then, on failure, block.
         * CANCELLED: This node is cancelled due to timeout or interrupt. Nodes never leave this state. In particular, a
         * thread with cancelled node never again blocks. CONDITION: This node is currently on a condition queue. It
         * will not be used as a sync queue node until transferred, at which time the status will be set to 0. (Use of
         * this value here has nothing to do with the other uses of the field, but simplifies mechanics.) PROPAGATE: A
         * releaseShared should be propagated to other nodes. This is set (for head node only) in doReleaseShared to
         * ensure propagation continues, even if other operations have since intervened. 0: None of the above
         *
         * The values are arranged numerically to simplify use. Non-negative values mean that a node doesn't need to
         * signal. So, most code doesn't need to check for particular values, just for sign.
         *
         * The field is initialized to 0 for normal sync nodes, and CONDITION for condition nodes. It is modified using
         * CAS (or when possible, unconditional volatile writes).
         * 标记当前节点的信号量状态 (1,0,‐1,‐2,‐3)5种状态
         * 使用CAS更改状态，volatile保证线程可见性，高并发场景下，
         * 即被一个线程修改后，状态会立马让其他线程可见。
         */
        volatile int waitStatus;

        /**
         * Link to predecessor node that current node/thread relies on for checking waitStatus. Assigned during
         * enqueuing, and nulled out (for sake of GC) only upon dequeuing. Also, upon cancellation of a predecessor, we
         * short-circuit while finding a non-cancelled one, which will always exist because the head node is never
         * cancelled: A node becomes head only as a result of successful acquire. A cancelled thread never succeeds in
         * acquiring, and a thread only cancels itself, not any other node.
         */
        volatile Node prev;

        /**
         * Link to the successor node that the current node/thread unparks upon release. Assigned during enqueuing,
         * adjusted when bypassing cancelled predecessors, and nulled out (for sake of GC) when dequeued. The enq
         * operation does not assign next field of a predecessor until after attachment, so seeing a null next field
         * does not necessarily mean that node is at end of queue. However, if a next field appears to be null, we can
         * scan prev's from the tail to double-check. The next field of cancelled nodes is set to point to the node
         * itself instead of null, to make life easier for isOnSyncQueue.
         */
        volatile Node next;

        /**
         * The thread that enqueued this node. Initialized on construction and nulled out after use.
         */
        volatile Thread thread;

        /**
         * Link to next node waiting on condition, or the special value SHARED. Because condition queues are accessed
         * only when holding in exclusive mode, we just need a simple linked queue to hold nodes while they are waiting
         * on conditions. They are then transferred to the queue to re-acquire. And because conditions can only be
         * exclusive, we save a field by using special value to indicate shared mode.
         * 等待队列中的后继节点，如果当前节点是共享的，那么这个字段是一个SHAR ED常量，也就是说节点类型(独占和共享)和等待队列中的后继节点共用同一个字段。
         * 注意这个这个前面的next节点不一样，这两个不是一个队列中的，这个是给condition状态下的等待队列使用的
         */
        Node nextWaiter;

        /**
         * Returns true if node is waiting in shared mode.
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        /**
         * Returns previous node, or throws NullPointerException if null. Use when predecessor cannot be null. The null
         * check could be elided, but is present to help the VM.
         *
         * @return the predecessor of this node
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }

        //空节点，用于标记共享模式
        Node() { // Used to establish initial head or SHARED marker
        }

        //用于同步队列CLH
        Node(Thread thread, Node mode) { // Used by addWaiter
            this.nextWaiter = mode;
            this.thread = thread;
        }
        //用于条件队列
        Node(Thread thread, int waitStatus) { // Used by Condition
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }

    /**
     * Head of the wait queue, lazily initialized. Except for initialization, it is modified only via method setHead.
     * Note: If head exists, its waitStatus is guaranteed not to be CANCELLED.
     */
    private transient volatile Node head;

    /**
     * Tail of the wait queue, lazily initialized. Modified only via method enq to add new wait node.
     */
    private transient volatile Node tail;

    /**
     * The synchronization state. 这是一个特别重要的属性，表示同步资源状态，当并发来临时，就是各个线程来争抢这个state
     */
    private volatile int state;

    /**
     * Returns the current value of synchronization state. This operation has memory semantics of a {@code volatile}
     * read.
     * 
     * @return current state value
     */
    protected final int getState() {
        return state;
    }

    /**
     * Sets the value of synchronization state. This operation has memory semantics of a {@code volatile} write.
     * 
     * @param newState
     *            the new state value
     */
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * Atomically sets synchronization state to the given updated value if the current state value equals the expected
     * value. This operation has memory semantics of a {@code volatile} read and write.
     *
     * @param expect
     *            the expected value
     * @param update
     *            the new value
     * @return {@code true} if successful. False return indicates that the actual value was not equal to the expected
     *         value.
     */
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

    // Queuing utilities

    /**
     * The number of nanoseconds for which it is faster to spin rather than to use timed park. A rough estimate suffices
     * to improve responsiveness with very short timeouts.
     */
    static final long spinForTimeoutThreshold = 1000L;

    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     * 
     * @param node
     *            the node to insert
     * @return node's predecessor 注意这里返回的是新节点的前一个节点
     * 入队操作，入CLH队列
     */
    private Node enq(final Node node) {
        // 直接一个死循环入队啊，因为下面的cas操作，会导致并发的情况下，入队失败，所以写个死循环，入队失败就继续争抢去
        for (;;) {
            Node t = tail;
            // 必须初始化，这里可以看到初始化的时候new了一个Node，但是没有入队，仅仅是设置了一个空的head，下一轮循环node才竞争入队
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                // 将node赋值给tail，然后tail指向了node,t还是原来的tail,所以下面再把old tail的next指向tail,最后返回的是新tail的pre
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

    /**
     * Creates and enqueues node for current thread and given mode.
     *
     * @param mode
     *            Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     * 将当前线程添加到等待队列，注意，这里返回的就是当前的节点，enq(Node)返回的是前驱节点
     */
    private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure，先尝试直接入队，如果入队失败了，再调用eng方法来阻塞入队，所以下面的快速入队中的方法跟上面enq中的方法差不多
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

    /**
     * Sets head of queue to be node, thus dequeuing. Called only by acquire methods. Also nulls out unused fields for
     * sake of GC and to suppress unnecessary signals and traversals.
     *
     * @param node
     *            the node
     */
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }

    /**
     * Wakes up node's successor, if one exists.
     *
     * @param node
     *            the node
     */
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        // 这些操作有点状态机的意思
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0); // 将等待状态waitStatus设置为初始值0

        /* 要取消unpark的线程被保留在继任者中，后者通常只是下一个节点。
         * 但如果取消或明显为null，则从tail向后遍历，以找到实际的未取消的后续对象。
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        // 若后继结点为空，或状态为CANCEL（已失效），则从后尾部往前遍历找到最前的一个处于正常阻塞状态的结点进行唤醒
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            // 唤醒线程
            LockSupport.unpark(s.thread);
    }

    /**
     * Release action for shared mode -- signals successor and ensures propagation. (Note: For exclusive mode, release
     * just amounts to calling unparkSuccessor of head if it needs signal.)
     */
    private void doReleaseShared() {
        /*
         * Ensure that a release propagates, even if there are other
         * in-progress acquires/releases.  This proceeds in the usual
         * way of trying to unparkSuccessor of head if it needs
         * signal. But if it does not, status is set to PROPAGATE to
         * ensure that upon release, propagation continues.
         * Additionally, we must loop in case a new node is added
         * while we are doing this. Also, unlike other uses of
         * unparkSuccessor, we need to know if CAS to reset status
         * fails, if so rechecking.
         */
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                // 头结点的状态
                if (ws == Node.SIGNAL) {
                    // 修改头结点的状态
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue; // loop to recheck cases
                    // 修改成功，就去唤醒继任者，应该就是下一个，release一个，唤醒一个
                    unparkSuccessor(h);
                } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue; // loop on failed CAS
            }
            if (h == head) // loop if head changed
                break;
        }
    }

    /**
     * Sets head of queue, and checks if successor may be waiting in shared mode, if so propagating if either propagate
     * > 0 or PROPAGATE status was set.
     *
     * @param node
     *            the node
     * @param propagate
     *            the return value from a tryAcquireShared
     */
    private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; // Record old head for check below
        setHead(node);
        /*
         * Try to signal next queued node if:
         *   Propagation was indicated by caller,
         *     or was recorded (as h.waitStatus either before
         *     or after setHead) by a previous operation
         *     (note: this uses sign-check of waitStatus because
         *      PROPAGATE status may transition to SIGNAL.)
         * and
         *   The next node is waiting in shared mode,
         *     or we don't know, because it appears null
         *
         * The conservatism in both of these checks may cause
         * unnecessary wake-ups, but only when there are multiple
         * racing acquires/releases, so most need signals now or soon
         * anyway.
         */
        if (propagate > 0 || h == null || h.waitStatus < 0 || (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }

    // Utilities for various versions of acquire

    /**
     * Cancels an ongoing attempt to acquire.
     *
     * @param node
     *            the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node == null)
            return;

        node.thread = null;

        // Skip cancelled predecessors 这就是顺带手的把所有node的前驱节点为cancel状态的node都给从队列中删除了
        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        // predNext is the apparent node to unsplice. CASes below will
        // fail if not, in which case, we lost race vs another cancel
        // or signal, so no further action is necessary.
        Node predNext = pred.next;

        // Can use unconditional write instead of CAS here.
        // After this atomic step, other Nodes can skip past us.
        // Before, we are free of interference from other threads.
        node.waitStatus = Node.CANCELLED;

        // If we are the tail, remove ourselves.
        if (node == tail && compareAndSetTail(node, pred)) {
            compareAndSetNext(pred, predNext, null);
        } else {
            // 看不懂啊看不懂
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
            int ws;
            if (pred != head && ((ws = pred.waitStatus) == Node.SIGNAL
                || (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) && pred.thread != null) {
                Node next = node.next;
                if (next != null && next.waitStatus <= 0)
                    compareAndSetNext(pred, predNext, next);
            } else {
                // 为啥只在这里进行unpark操作，只有进入这个分支才会unpark
                unparkSuccessor(node);
            }

            node.next = node; // help GC
        }
    }

    /**
     * Checks and updates status for a node that failed to acquire. Returns true if thread should block. This is the
     * main signal control in all acquire loops. Requires that pred == node.prev.
     *
     * 校验并且更新node的状态，该node是acquire失败的node，返回值表示线程是否应该阻塞，这个方法也是主要的信号控制方式在所有的acquire循环中
     *
     * @param pred
     *            node's predecessor holding status
     * @param node
     *            the node
     * @return {@code true} if thread should block
     * 这个方法到底几个意思？判断是否这个节点是否阻塞，用的是前驱节点的状态是否是SIGNAL，如果是SIGNAL就返回true，其他的都return false
     * 返回false就意味着，调用方还不能让线程阻塞，还要继续去acquire？
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        // 注意，这里是找前驱节点的waitStatus
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             *
             * 这个节点已经设置了请求释放的状态来发出信号，这样它就可以安全阻塞了。
             */
            return true;
        if (ws > 0) {
            /*static final int CANCELLED = 1;只有取消状态>0，其他都是<0的
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             * 前驱节点被取消了，跳过前驱节点(predecessors为啥是负数，还有好多前驱？)，并重试
             */
            do {
                // 我勒个去，还真是，因为这个节点前面可能一堆节点都是cancel状态，一直往前，直到一个非cancel状态的node
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /* CONDITION = -2;PROPAGATE = -3;还有这两种状态没有参与判断，但是下面的注释中已经说了waitStatus只能为0或者-3，这是为啥
             * 表示我们需要一个Signal，但是还不用阻塞，调用者可能是需要retry来保障在阻塞前不可能acquire成功
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             * 给前置节点设置状态为SIGNAL：当前驱节点waitStatus为 0 or PROPAGATE状态时，将其设置为SIGNAL状态，然后当前结点才可以可以被安全地park
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

    /**
     * Convenience method to interrupt current thread.
     */
    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

    /**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        // 传入的this是当前的对象，用于记录该线程的阻塞对象，这里park了，是不是代码就不往下走了？
        LockSupport.park(this);
        return Thread.interrupted();
    }

    /*
     * Various flavors of acquire, varying in exclusive/shared and
     * control modes.  Each is mostly the same, but annoyingly
     * different.  Only a little bit of factoring is possible due to
     * interactions of exception mechanics (including ensuring that we
     * cancel if tryAcquire throws exception) and other control, at
     * least not without hurting performance too much.
     */

    /**
     * Acquires in exclusive uninterruptible mode for thread already in queue. Used by condition wait methods as well as
     * acquire.
     * 以独占【不可中断】模式获取已经在队列中的线程。用于条件等待方法和acquire方法。
     * @param node
     *            the node
     * @param arg
     *            the acquire argument
     * @return {@code true} if interrupted while waiting
     *
     * 已经在队列当中的Thread节点，准备阻塞等待获取锁
     */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            // 注意这里是死循环，但凡是抢占锁的地方，基本都是死循环
            // 有意思了，就是所有队列中的节点从丢进来，调用这个方法开始，就是在尝试获取锁，只有获取到了锁，才能从死循环解放出来，就是第一个if的return
            // 如果没有拿到锁，那么就在第二个if分支中被阻塞，也就是parkAndCheckInterrupt()，阻塞了之后只有通过中断或者unpark来唤醒
            // 唤醒了之后继续开始tryAcquire，直到拿到位置，不死不休，所以这里也记录了最后被唤醒的方式是不是中断，通过返回值传递出去
            // 过了死循环以后，最后取消这个节点的trayAcquire操作，应该就是出队等操作。
            // 经过上面分析，当节点抢到了锁之后，并且执行完自己的逻辑之后，肯定得去主动释放，释放的时候触发队列中其他的被阻塞的下一个node来trayAcquire
            for (;;) {
                // 找前驱节点，如果前驱节点就是头结点，说明在执行代码的这段时间已经可以去抢占锁资源了，所以直接就去tryAcquire
                final Node p = node.predecessor();
                // 如果前驱结点是头结点，才tryAcquire，其他结点是没有机会tryAcquire的。
                if (p == head && tryAcquire(arg)) {
                    // 只有从队列中抢占到资源的节点才可以设置Head为当前node
                    setHead(node);
                    // 原来的head丢弃
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                // 1 第一个方法判断的是在fail acquire之后是否阻塞，只有前驱节点是Signal状态的才阻塞
                // 2 如果判断应该阻塞，则进入第二个方法是阻塞当前线程（底层阻塞住了，后面的代码应该不执行了，等待被unpark和中断，是不是因为这样才要check中断）
                // 并且返回当前线程是否被中断，因为被中断，就直接解锁了，if分支里面的代码才可以走
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            // failed表示acquire失败了，从上面的代码上来看，failed是初始值为true，只有抢占到锁之后才会被赋值为false，其他地方不会对这个只有改变
            // 而且，如果不是抢占到了锁，return之后，想进入这个分支，必然是因为发生了异常，所以才会cancel acquire吧
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in exclusive interruptible mode.
     * 
     * @param arg
     *            the acquire argument
     */
    private void doAcquireInterruptibly(int arg) throws InterruptedException {
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in exclusive timed mode.
     *
     * @param arg
     *            the acquire argument
     * @param nanosTimeout
     *            max wait time
     * @return {@code true} if acquired
     */
    private boolean doAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return true;
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in shared uninterruptible mode.
     * 
     * @param arg
     *            the acquire argument
     */
    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in shared interruptible mode.
     * 
     * @param arg
     *            the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg) throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in shared timed mode.
     *
     * @param arg
     *            the acquire argument
     * @param nanosTimeout
     *            max wait time
     * @return {@code true} if acquired
     */
    private boolean doAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return true;
                    }
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    // Main exported methods
    /**
     * 获取资源的方式定义了两种
     * 独占exclusive mode：只有一个线程能拿到资源 ReentrantLock
     * 共享shared mode:可以多个线程拿到资源 Semaphore/CountDownLatch
     */
    /**
     * 以独占的方式来争抢资源 Attempts to acquire in exclusive mode. This method should query if the state of the object permits it
     * to be acquired in the exclusive mode, and if so to acquire it.
     *
     * <p>
     * This method is always invoked by the thread performing acquire. If this method reports failure, the acquire
     * method may queue the thread, if it is not already queued, until it is signalled by a release from some other
     * thread. This can be used to implement method {@link Lock#tryLock()}.
     *
     * <p>
     * The default implementation throws {@link UnsupportedOperationException}.
     *
     * @param arg
     *            the acquire argument. This value is always the one passed to an acquire method, or is the value saved
     *            on entry to a condition wait. The value is otherwise uninterpreted and can represent anything you
     *            like.
     * @return {@code true} if successful. Upon success, this object has been acquired.
     * @throws IllegalMonitorStateException
     *             if acquiring would place this synchronizer in an illegal state. This exception must be thrown in a
     *             consistent fashion for synchronization to work correctly.
     * @throws UnsupportedOperationException
     *             if exclusive mode is not supported
     */
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * 跟上面的tryAcquire是一对，释放以独占的方式获取到的资源 Attempts to set the state to reflect a release in exclusive mode.
     *
     * <p>
     * This method is always invoked by the thread performing release.
     *
     * <p>
     * The default implementation throws {@link UnsupportedOperationException}.
     *
     * @param arg
     *            the release argument. This value is always the one passed to a release method, or the current state
     *            value upon entry to a condition wait. The value is otherwise uninterpreted and can represent anything
     *            you like.
     * @return {@code true} if this object is now in a fully released state, so that any waiting threads may attempt to
     *         acquire; and {@code false} otherwise.
     * @throws IllegalMonitorStateException
     *             if releasing would place this synchronizer in an illegal state. This exception must be thrown in a
     *             consistent fashion for synchronization to work correctly.
     * @throws UnsupportedOperationException
     *             if exclusive mode is not supported
     */
    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * 共享方式。尝试获取资源。负数表示失败；0表示成功，但没有剩余可用资源；正数表示成功，且有剩余资源
     * Attempts to acquire in shared mode. This method should query if the state of the object permits it to be acquired
     * in the shared mode, and if so to acquire it.
     *
     * <p>
     * This method is always invoked by the thread performing acquire. If this method reports failure, the acquire
     * method may queue the thread, if it is not already queued, until it is signalled by a release from some other
     * thread.
     *
     * <p>
     * The default implementation throws {@link UnsupportedOperationException}.
     *
     * @param arg
     *            the acquire argument. This value is always the one passed to an acquire method, or is the value saved
     *            on entry to a condition wait. The value is otherwise uninterpreted and can represent anything you
     *            like.
     * @return a negative value on failure; zero if acquisition in shared mode succeeded but no subsequent shared-mode
     *         acquire can succeed; and a positive value if acquisition in shared mode succeeded and subsequent
     *         shared-mode acquires might also succeed, in which case a subsequent waiting thread must check
     *         availability. (Support for three different return values enables this method to be used in contexts where
     *         acquires only sometimes act exclusively.) Upon success, this object has been acquired.
     * @throws IllegalMonitorStateException
     *             if acquiring would place this synchronizer in an illegal state. This exception must be thrown in a
     *             consistent fashion for synchronization to work correctly.
     * @throws UnsupportedOperationException
     *             if shared mode is not supported
     */
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * 共享方式。尝试释放资源，如果释放后允许唤醒后续等待结点返回true，否则返回false
     * Attempts to set the state to reflect a release in shared mode.
     *
     * <p>
     * This method is always invoked by the thread performing release.
     *
     * <p>
     * The default implementation throws {@link UnsupportedOperationException}.
     *
     * @param arg
     *            the release argument. This value is always the one passed to a release method, or the current state
     *            value upon entry to a condition wait. The value is otherwise uninterpreted and can represent anything
     *            you like.
     * @return {@code true} if this release of shared mode may permit a waiting acquire (shared or exclusive) to
     *         succeed; and {@code false} otherwise
     * @throws IllegalMonitorStateException
     *             if releasing would place this synchronizer in an illegal state. This exception must be thrown in a
     *             consistent fashion for synchronization to work correctly.
     * @throws UnsupportedOperationException
     *             if shared mode is not supported
     */
    protected boolean tryReleaseShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * Returns {@code true} if synchronization is held exclusively with respect to the current (calling) thread. This
     * method is invoked upon each call to a non-waiting {@link ConditionObject} method. (Waiting methods instead invoke
     * {@link #release}.)
     *
     * <p>
     * The default implementation throws {@link UnsupportedOperationException}. This method is invoked internally only
     * within {@link ConditionObject} methods, so need not be defined if conditions are not used.
     *
     * @return {@code true} if synchronization is held exclusively; {@code false} otherwise
     * @throws UnsupportedOperationException
     *             if conditions are not supported
     */
    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }

    /**
     * 这是获取独占锁，只要方法上没有加shared就是默认独占模式
     * Acquires in exclusive mode, ignoring interrupts. Implemented by invoking at least once {@link #tryAcquire},
     * returning on success. Otherwise the thread is queued, possibly repeatedly blocking and unblocking, invoking
     * {@link #tryAcquire} until success. This method can be used to implement method {@link Lock#lock}.
     *
     * @param arg
     *            the acquire argument. This value is conveyed to {@link #tryAcquire} but is otherwise uninterpreted and
     *            can represent anything you like.
     */
    public final void acquire(int arg) {
        // 1 tryAcquire尝试去抢锁，如果tryAcquire失败了，则继续走 && 后面的acquireQueued,注意这个tryAcquire交给子类实现，每种锁去争抢资源的方式不一样
        // 2 acquireQueued（）方法中还调用了addWaiter(Node.EXCLUSIVE)方法：该方法就是把这个节点直接添加到CLH等待队列中去，并且返回了当前节点，注意注意，addWaiter仅仅是添加了一个节点到队列中，设置了mode，但是没有设置ownerThread
        // 3 acquireQueued（）方法是指在队列中的线程去try acquire锁,如果没有acquire到锁，那么就阻塞在那里，直到acquire到锁才会返回结果，返回值为线程的中断状态
        // 4 acquireQueued（）的返回值表示
        if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            // 这很迷惑啊，上面先直接tryAcquire失败了，然后进队列去进行tryAcquire，如果acquire到了，但是是被中断唤醒的，还要selfInterrupt?这是几个意思？虽然也没啥影响
            selfInterrupt();
    }

    /**
     * Acquires in exclusive mode, aborting if interrupted. Implemented by first checking interrupt status, then
     * invoking at least once {@link #tryAcquire}, returning on success. Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link #tryAcquire} until success or the thread is interrupted. This
     * method can be used to implement method {@link Lock#lockInterruptibly}.
     *
     * @param arg
     *            the acquire argument. This value is conveyed to {@link #tryAcquire} but is otherwise uninterpreted and
     *            can represent anything you like.
     * @throws InterruptedException
     *             if the current thread is interrupted
     */
    public final void acquireInterruptibly(int arg) throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }

    /**
     * Attempts to acquire in exclusive mode, aborting if interrupted, and failing if the given timeout elapses.
     * Implemented by first checking interrupt status, then invoking at least once {@link #tryAcquire}, returning on
     * success. Otherwise, the thread is queued, possibly repeatedly blocking and unblocking, invoking
     * {@link #tryAcquire} until success or the thread is interrupted or the timeout elapses. This method can be used to
     * implement method {@link Lock#tryLock(long, TimeUnit)}.
     *
     * @param arg
     *            the acquire argument. This value is conveyed to {@link #tryAcquire} but is otherwise uninterpreted and
     *            can represent anything you like.
     * @param nanosTimeout
     *            the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException
     *             if the current thread is interrupted
     */
    public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquire(arg) || doAcquireNanos(arg, nanosTimeout);
    }

    /**
     * Releases in exclusive mode. Implemented by unblocking one or more threads if {@link #tryRelease} returns true.
     * This method can be used to implement method {@link Lock#unlock}.
     *
     * @param arg
     *            the release argument. This value is conveyed to {@link #tryRelease} but is otherwise uninterpreted and
     *            can represent anything you like.
     * @return the value returned from {@link #tryRelease}
     */
    public final boolean release(int arg) {
        // tryRelease交给子类去实现,子类设置成功会把state=0，ownerThread = null
        if (tryRelease(arg)) {
            Node h = head;
            // 这又是啥操作？：是否还记得队列中的线程都还在阻塞状态中呢，这就是在unpark队列中的线程来tryAcquire
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

    /**
     * Acquires in shared mode, ignoring interrupts. Implemented by first invoking at least once
     * {@link #tryAcquireShared}, returning on success. Otherwise the thread is queued, possibly repeatedly blocking and
     * unblocking, invoking {@link #tryAcquireShared} until success.
     *
     * @param arg
     *            the acquire argument. This value is conveyed to {@link #tryAcquireShared} but is otherwise
     *            uninterpreted and can represent anything you like.
     */
    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }

    /**
     * Acquires in shared mode, aborting if interrupted. Implemented by first checking interrupt status, then invoking
     * at least once {@link #tryAcquireShared}, returning on success. Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link #tryAcquireShared} until success or the thread is
     * interrupted.
     * 
     * @param arg
     *            the acquire argument. This value is conveyed to {@link #tryAcquireShared} but is otherwise
     *            uninterpreted and can represent anything you like.
     * @throws InterruptedException
     *             if the current thread is interrupted
     */
    public final void acquireSharedInterruptibly(int arg) throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

    /**
     * Attempts to acquire in shared mode, aborting if interrupted, and failing if the given timeout elapses.
     * Implemented by first checking interrupt status, then invoking at least once {@link #tryAcquireShared}, returning
     * on success. Otherwise, the thread is queued, possibly repeatedly blocking and unblocking, invoking
     * {@link #tryAcquireShared} until success or the thread is interrupted or the timeout elapses.
     *
     * @param arg
     *            the acquire argument. This value is conveyed to {@link #tryAcquireShared} but is otherwise
     *            uninterpreted and can represent anything you like.
     * @param nanosTimeout
     *            the maximum number of nanoseconds to wait
     * @return {@code true} if acquired; {@code false} if timed out
     * @throws InterruptedException
     *             if the current thread is interrupted
     */
    public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquireShared(arg) >= 0 || doAcquireSharedNanos(arg, nanosTimeout);
    }

    /**
     * Releases in shared mode. Implemented by unblocking one or more threads if {@link #tryReleaseShared} returns true.
     *
     * @param arg
     *            the release argument. This value is conveyed to {@link #tryReleaseShared} but is otherwise
     *            uninterpreted and can represent anything you like.
     * @return the value returned from {@link #tryReleaseShared}
     */
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

    // Queue inspection methods

    /**
     * Queries whether any threads are waiting to acquire. Note that because cancellations due to interrupts and
     * timeouts may occur at any time, a {@code true} return does not guarantee that any other thread will ever acquire.
     *
     * <p>
     * In this implementation, this operation returns in constant time.
     *
     * @return {@code true} if there may be other threads waiting to acquire
     */
    public final boolean hasQueuedThreads() {
        return head != tail;
    }

    /**
     * Queries whether any threads have ever contended to acquire this synchronizer; that is if an acquire method has
     * ever blocked.
     *
     * <p>
     * In this implementation, this operation returns in constant time.
     *
     * @return {@code true} if there has ever been contention
     */
    public final boolean hasContended() {
        return head != null;
    }

    /**
     * Returns the first (longest-waiting) thread in the queue, or {@code null} if no threads are currently queued.
     *
     * <p>
     * In this implementation, this operation normally returns in constant time, but may iterate upon contention if
     * other threads are concurrently modifying the queue.
     *
     * @return the first (longest-waiting) thread in the queue, or {@code null} if no threads are currently queued
     */
    public final Thread getFirstQueuedThread() {
        // handle only fast path, else relay
        return (head == tail) ? null : fullGetFirstQueuedThread();
    }

    /**
     * Version of getFirstQueuedThread called when fastpath fails
     */
    private Thread fullGetFirstQueuedThread() {
        /*
         * The first node is normally head.next. Try to get its
         * thread field, ensuring consistent reads: If thread
         * field is nulled out or s.prev is no longer head, then
         * some other thread(s) concurrently performed setHead in
         * between some of our reads. We try this twice before
         * resorting to traversal.
         */
        Node h, s;
        Thread st;
        if (((h = head) != null && (s = h.next) != null && s.prev == head && (st = s.thread) != null)
            || ((h = head) != null && (s = h.next) != null && s.prev == head && (st = s.thread) != null))
            return st;

        /*
         * Head's next field might not have been set yet, or may have
         * been unset after setHead. So we must check to see if tail
         * is actually first node. If not, we continue on, safely
         * traversing from tail back to head to find first,
         * guaranteeing termination.
         */

        Node t = tail;
        Thread firstThread = null;
        while (t != null && t != head) {
            Thread tt = t.thread;
            if (tt != null)
                firstThread = tt;
            t = t.prev;
        }
        return firstThread;
    }

    /**
     * Returns true if the given thread is currently queued.
     *
     * <p>
     * This implementation traverses the queue to determine presence of the given thread.
     *
     * @param thread
     *            the thread
     * @return {@code true} if the given thread is on the queue
     * @throws NullPointerException
     *             if the thread is null
     */
    public final boolean isQueued(Thread thread) {
        if (thread == null)
            throw new NullPointerException();
        for (Node p = tail; p != null; p = p.prev)
            if (p.thread == thread)
                return true;
        return false;
    }

    /**
     * Returns {@code true} if the apparent first queued thread, if one exists, is waiting in exclusive mode. If this
     * method returns {@code true}, and the current thread is attempting to acquire in shared mode (that is, this method
     * is invoked from {@link #tryAcquireShared}) then it is guaranteed that the current thread is not the first queued
     * thread. Used only as a heuristic in ReentrantReadWriteLock.
     */
    final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) != null && (s = h.next) != null && !s.isShared() && s.thread != null;
    }

    /**
     * Queries whether any threads have been waiting to acquire longer than the current thread.
     *
     * <p>
     * An invocation of this method is equivalent to (but may be more efficient than):
     * 
     * <pre>
     *  {@code
     * getFirstQueuedThread() != Thread.currentThread() &&
     * hasQueuedThreads()}
     * </pre>
     *
     * <p>
     * Note that because cancellations due to interrupts and timeouts may occur at any time, a {@code true} return does
     * not guarantee that some other thread will acquire before the current thread. Likewise, it is possible for another
     * thread to win a race to enqueue after this method has returned {@code false}, due to the queue being empty.
     *
     * <p>
     * This method is designed to be used by a fair synchronizer to avoid
     * <a href="AbstractQueuedSynchronizer#barging">barging</a>. Such a synchronizer's {@link #tryAcquire} method should
     * return {@code false}, and its {@link #tryAcquireShared} method should return a negative value, if this method
     * returns {@code true} (unless this is a reentrant acquire). For example, the {@code
     * tryAcquire} method for a fair, reentrant, exclusive mode synchronizer might look like this:
     *
     * <pre>
     *  {@code
     * protected boolean tryAcquire(int arg) {
     *   if (isHeldExclusively()) {
     *     // A reentrant acquire; increment hold count
     *     return true;
     *   } else if (hasQueuedPredecessors()) {
     *     return false;
     *   } else {
     *     // try to acquire normally
     *   }
     * }}
     * </pre>
     *
     * @return {@code true} if there is a queued thread preceding the current thread, and {@code false} if the current
     *         thread is at the head of the queue or the queue is empty
     * @since 1.7
     */
    public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        return h != t && ((s = h.next) == null || s.thread != Thread.currentThread());
    }

    // Instrumentation and monitoring methods

    /**
     * Returns an estimate of the number of threads waiting to acquire. The value is only an estimate because the number
     * of threads may change dynamically while this method traverses internal data structures. This method is designed
     * for use in monitoring system state, not for synchronization control.
     *
     * @return the estimated number of threads waiting to acquire
     */
    public final int getQueueLength() {
        int n = 0;
        for (Node p = tail; p != null; p = p.prev) {
            if (p.thread != null)
                ++n;
        }
        return n;
    }

    /**
     * Returns a collection containing threads that may be waiting to acquire. Because the actual set of threads may
     * change dynamically while constructing this result, the returned collection is only a best-effort estimate. The
     * elements of the returned collection are in no particular order. This method is designed to facilitate
     * construction of subclasses that provide more extensive monitoring facilities.
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            Thread t = p.thread;
            if (t != null)
                list.add(t);
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to acquire in exclusive mode. This has the same
     * properties as {@link #getQueuedThreads} except that it only returns those threads waiting due to an exclusive
     * acquire.
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getExclusiveQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (!p.isShared()) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a collection containing threads that may be waiting to acquire in shared mode. This has the same
     * properties as {@link #getQueuedThreads} except that it only returns those threads waiting due to a shared
     * acquire.
     *
     * @return the collection of threads
     */
    public final Collection<Thread> getSharedQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (p.isShared()) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * Returns a string identifying this synchronizer, as well as its state. The state, in brackets, includes the String
     * {@code "State ="} followed by the current value of {@link #getState}, and either {@code "nonempty"} or
     * {@code "empty"} depending on whether the queue is empty.
     *
     * @return a string identifying this synchronizer, as well as its state
     */
    public String toString() {
        int s = getState();
        String q = hasQueuedThreads() ? "non" : "";
        return super.toString() + "[State = " + s + ", " + q + "empty queue]";
    }

    // Internal support methods for Conditions

    /**
     * Returns true if a node, always one that was initially placed on a condition queue, is now waiting to reacquire on
     * sync queue.
     * 
     * @param node
     *            the node
     * @return true if is reacquiring
     */
    final boolean isOnSyncQueue(Node node) {
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
        if (node.next != null) // If has successor, it must be on queue
            return true;
        /*
         * node.prev can be non-null, but not yet on queue because
         * the CAS to place it on queue can fail. So we have to
         * traverse from tail to make sure it actually made it.  It
         * will always be near the tail in calls to this method, and
         * unless the CAS failed (which is unlikely), it will be
         * there, so we hardly ever traverse much.
         */
        return findNodeFromTail(node);
    }

    /**
     * Returns true if node is on sync queue by searching backwards from tail. Called only when needed by isOnSyncQueue.
     * 
     * @return true if present
     */
    private boolean findNodeFromTail(Node node) {
        Node t = tail;
        for (;;) {
            if (t == node)
                return true;
            if (t == null)
                return false;
            t = t.prev;
        }
    }

    /**
     * Transfers a node from a condition queue onto sync queue. Returns true if successful.
     * 
     * @param node
     *            the node
     * @return true if successfully transferred (else the node was cancelled before signal)
     */
    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;

        /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         */
        Node p = enq(node);
        int ws = p.waitStatus;
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }

    /**
     * Transfers node, if necessary, to sync queue after a cancelled wait. Returns true if thread was cancelled before
     * being signalled.
     *
     * @param node
     *            the node
     * @return true if cancelled before the node was signalled
     */
    final boolean transferAfterCancelledWait(Node node) {
        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            enq(node);
            return true;
        }
        /*
         * If we lost out to a signal(), then we can't proceed
         * until it finishes its enq().  Cancelling during an
         * incomplete transfer is both rare and transient, so just
         * spin.
         */
        while (!isOnSyncQueue(node))
            Thread.yield();
        return false;
    }

    /**
     * Invokes release with current state value; returns saved state. Cancels node and throws exception on failure.
     * 
     * @param node
     *            the condition node for this wait
     * @return previous sync state
     */
    final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            int savedState = getState();
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
                throw new IllegalMonitorStateException();
            }
        } finally {
            if (failed)
                node.waitStatus = Node.CANCELLED;
        }
    }

    // Instrumentation methods for conditions

    /**
     * Queries whether the given ConditionObject uses this synchronizer as its lock.
     *
     * @param condition
     *            the condition
     * @return {@code true} if owned
     * @throws NullPointerException
     *             if the condition is null
     */
    public final boolean owns(ConditionObject condition) {
        return condition.isOwnedBy(this);
    }

    /**
     * Queries whether any threads are waiting on the given condition associated with this synchronizer. Note that
     * because timeouts and interrupts may occur at any time, a {@code true} return does not guarantee that a future
     * {@code signal} will awaken any threads. This method is designed primarily for use in monitoring of the system
     * state.
     *
     * @param condition
     *            the condition
     * @return {@code true} if there are any waiting threads
     * @throws IllegalMonitorStateException
     *             if exclusive synchronization is not held
     * @throws IllegalArgumentException
     *             if the given condition is not associated with this synchronizer
     * @throws NullPointerException
     *             if the condition is null
     */
    public final boolean hasWaiters(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.hasWaiters();
    }

    /**
     * Returns an estimate of the number of threads waiting on the given condition associated with this synchronizer.
     * Note that because timeouts and interrupts may occur at any time, the estimate serves only as an upper bound on
     * the actual number of waiters. This method is designed for use in monitoring of the system state, not for
     * synchronization control.
     *
     * @param condition
     *            the condition
     * @return the estimated number of waiting threads
     * @throws IllegalMonitorStateException
     *             if exclusive synchronization is not held
     * @throws IllegalArgumentException
     *             if the given condition is not associated with this synchronizer
     * @throws NullPointerException
     *             if the condition is null
     */
    public final int getWaitQueueLength(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitQueueLength();
    }

    /**
     * Returns a collection containing those threads that may be waiting on the given condition associated with this
     * synchronizer. Because the actual set of threads may change dynamically while constructing this result, the
     * returned collection is only a best-effort estimate. The elements of the returned collection are in no particular
     * order.
     *
     * @param condition
     *            the condition
     * @return the collection of threads
     * @throws IllegalMonitorStateException
     *             if exclusive synchronization is not held
     * @throws IllegalArgumentException
     *             if the given condition is not associated with this synchronizer
     * @throws NullPointerException
     *             if the condition is null
     */
    public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitingThreads();
    }

    /**
     * Condition implementation for a {@link AbstractQueuedSynchronizer} serving as the basis of a {@link Lock}
     * implementation.
     *
     * <p>
     * Method documentation for this class describes mechanics, not behavioral specifications from the point of view of
     * Lock and Condition users. Exported versions of this class will in general need to be accompanied by documentation
     * describing condition semantics that rely on those of the associated {@code AbstractQueuedSynchronizer}.
     *
     * <p>
     * This class is Serializable, but all fields are transient, so deserialized conditions have no waiters.
     */
    public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /** First node of condition queue. */
        private transient Node firstWaiter;
        /** Last node of condition queue. */
        private transient Node lastWaiter;

        /**
         * Creates a new {@code ConditionObject} instance.
         */
        public ConditionObject() {}

        // Internal methods

        /**
         * Adds a new waiter to wait queue.
         * 
         * @return its new wait node
         */
        private Node addConditionWaiter() {
            Node t = lastWaiter;
            // If lastWaiter is cancelled, clean out.这就是边界判断之类的，判断是不是无效队列吧
            if (t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            // 这里是核心，构造函数的参数是waitStatus,底层还涉及到了Node对象的第三个核心指针nextWaiter（其他两个是pre
            // next），注意，从始至终，这个Node并没有进CLH队列，而是进了这个条件队列
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            if (t == null)
                firstWaiter = node;
            else
                t.nextWaiter = node;
            lastWaiter = node;
            // 通过firstWaiter和lastWaiter来记录条件队列中的头尾
            return node;
        }

        /**
         * Removes and transfers nodes until hit non-cancelled one or null. Split out from signal in part to encourage
         * compilers to inline the case of no waiters.
         * 
         * @param first
         *            (non-null) the first node on condition queue
         */
        private void doSignal(Node first) {
            do {
                if ((firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) && (first = firstWaiter) != null);
        }

        /**
         * Removes and transfers all nodes.
         * 
         * @param first
         *            (non-null) the first node on condition queue
         */
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }

        /**
         * Unlinks cancelled waiter nodes from condition queue. Called only while holding lock. This is called when
         * cancellation occurred during condition wait, and upon insertion of a new waiter when lastWaiter is seen to
         * have been cancelled. This method is needed to avoid garbage retention in the absence of signals. So even
         * though it may require a full traversal, it comes into play only when timeouts or cancellations occur in the
         * absence of signals. It traverses all nodes rather than stopping at a particular target to unlink all pointers
         * to garbage nodes without requiring many re-traversals during cancellation storms.
         */
        private void unlinkCancelledWaiters() {
            Node t = firstWaiter;
            Node trail = null;
            while (t != null) {
                Node next = t.nextWaiter;
                if (t.waitStatus != Node.CONDITION) {
                    t.nextWaiter = null;
                    if (trail == null)
                        firstWaiter = next;
                    else
                        trail.nextWaiter = next;
                    if (next == null)
                        lastWaiter = trail;
                } else
                    trail = t;
                t = next;
            }
        }

        // public methods

        /**
         * Moves the longest-waiting thread, if one exists, from the wait queue for this condition to the wait queue for
         * the owning lock.
         *
         * @throws IllegalMonitorStateException
         *             if {@link #isHeldExclusively} returns {@code false}
         */
        public final void signal() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignal(first);
        }

        /**
         * Moves all threads from the wait queue for this condition to the wait queue for the owning lock.
         *
         * @throws IllegalMonitorStateException
         *             if {@link #isHeldExclusively} returns {@code false}
         */
        public final void signalAll() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignalAll(first);
        }

        /**
         * Implements uninterruptible condition wait.
         * <ol>
         * <li>Save lock state returned by {@link #getState}.
         * <li>Invoke {@link #release} with saved state as argument, throwing IllegalMonitorStateException if it fails.
         * <li>Block until signalled.
         * <li>Reacquire by invoking specialized version of {@link #acquire} with saved state as argument.
         * </ol>
         */
        public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean interrupted = false;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if (Thread.interrupted())
                    interrupted = true;
            }
            if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }

        /*
         * For interruptible waits, we need to track whether to throw
         * InterruptedException, if interrupted while blocked on
         * condition, versus reinterrupt current thread, if
         * interrupted while blocked waiting to re-acquire.
         */

        /** Mode meaning to reinterrupt on exit from wait */
        private static final int REINTERRUPT = 1;
        /** Mode meaning to throw InterruptedException on exit from wait */
        private static final int THROW_IE = -1;

        /**
         * Checks for interrupt, returning THROW_IE if interrupted before signalled, REINTERRUPT if after signalled, or
         * 0 if not interrupted.
         */
        private int checkInterruptWhileWaiting(Node node) {
            return Thread.interrupted() ? (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 0;
        }

        /**
         * Throws InterruptedException, reinterrupts current thread, or does nothing, depending on mode.
         */
        private void reportInterruptAfterWait(int interruptMode) throws InterruptedException {
            if (interruptMode == THROW_IE)
                throw new InterruptedException();
            else if (interruptMode == REINTERRUPT)
                selfInterrupt();
        }

        /**
         * Implements interruptible condition wait.
         * <ol>
         * <li>If current thread is interrupted, throw InterruptedException.
         * <li>Save lock state returned by {@link #getState}.
         * <li>Invoke {@link #release} with saved state as argument, throwing IllegalMonitorStateException if it fails.
         * <li>Block until signalled or interrupted.
         * <li>Reacquire by invoking specialized version of {@link #acquire} with saved state as argument.
         * <li>If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        public final void await() throws InterruptedException {
            // 首先判断线程是否被中断，如果发生中断，直接抛异常，这就区别于awaitInterrupted
            if (Thread.interrupted())
                throw new InterruptedException();
            // 条件等待队列，并没有进入CLH队列
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

        /**
         * Implements timed condition wait.
         * <ol>
         * <li>If current thread is interrupted, throw InterruptedException.
         * <li>Save lock state returned by {@link #getState}.
         * <li>Invoke {@link #release} with saved state as argument, throwing IllegalMonitorStateException if it fails.
         * <li>Block until signalled, interrupted, or timed out.
         * <li>Reacquire by invoking specialized version of {@link #acquire} with saved state as argument.
         * <li>If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        public final long awaitNanos(long nanosTimeout) throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return deadline - System.nanoTime();
        }

        /**
         * Implements absolute timed condition wait.
         * <ol>
         * <li>If current thread is interrupted, throw InterruptedException.
         * <li>Save lock state returned by {@link #getState}.
         * <li>Invoke {@link #release} with saved state as argument, throwing IllegalMonitorStateException if it fails.
         * <li>Block until signalled, interrupted, or timed out.
         * <li>Reacquire by invoking specialized version of {@link #acquire} with saved state as argument.
         * <li>If interrupted while blocked in step 4, throw InterruptedException.
         * <li>If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        public final boolean awaitUntil(Date deadline) throws InterruptedException {
            long abstime = deadline.getTime();
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (System.currentTimeMillis() > abstime) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                LockSupport.parkUntil(this, abstime);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }

        /**
         * Implements timed condition wait.
         * <ol>
         * <li>If current thread is interrupted, throw InterruptedException.
         * <li>Save lock state returned by {@link #getState}.
         * <li>Invoke {@link #release} with saved state as argument, throwing IllegalMonitorStateException if it fails.
         * <li>Block until signalled, interrupted, or timed out.
         * <li>Reacquire by invoking specialized version of {@link #acquire} with saved state as argument.
         * <li>If interrupted while blocked in step 4, throw InterruptedException.
         * <li>If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        public final boolean await(long time, TimeUnit unit) throws InterruptedException {
            long nanosTimeout = unit.toNanos(time);
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }

        // support for instrumentation

        /**
         * Returns true if this condition was created by the given synchronization object.
         *
         * @return {@code true} if owned
         */
        final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
            return sync == AbstractQueuedSynchronizer.this;
        }

        /**
         * Queries whether any threads are waiting on this condition. Implements
         * {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
         *
         * @return {@code true} if there are any waiting threads
         * @throws IllegalMonitorStateException
         *             if {@link #isHeldExclusively} returns {@code false}
         */
        protected final boolean hasWaiters() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    return true;
            }
            return false;
        }

        /**
         * Returns an estimate of the number of threads waiting on this condition. Implements
         * {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
         *
         * @return the estimated number of waiting threads
         * @throws IllegalMonitorStateException
         *             if {@link #isHeldExclusively} returns {@code false}
         */
        protected final int getWaitQueueLength() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int n = 0;
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    ++n;
            }
            return n;
        }

        /**
         * Returns a collection containing those threads that may be waiting on this Condition. Implements
         * {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
         *
         * @return the collection of threads
         * @throws IllegalMonitorStateException
         *             if {@link #isHeldExclusively} returns {@code false}
         */
        protected final Collection<Thread> getWaitingThreads() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION) {
                    Thread t = w.thread;
                    if (t != null)
                        list.add(t);
                }
            }
            return list;
        }
    }

    /**
     * Setup to support compareAndSet. We need to natively implement this here: For the sake of permitting future
     * enhancements, we cannot explicitly subclass AtomicInteger, which would be efficient and useful otherwise. So, as
     * the lesser of evils, we natively implement using hotspot intrinsics API. And while we are at it, we do the same
     * for other CASable fields (which could otherwise be done with atomic field updaters).
     */
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long stateOffset;
    private static final long headOffset;
    private static final long tailOffset;
    private static final long waitStatusOffset;
    private static final long nextOffset;

    static {
        try {
            stateOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
            headOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
            tailOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
            waitStatusOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("waitStatus"));
            nextOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("next"));

        } catch (Exception ex) {
            throw new Error(ex);
        }
    }

    /**
     * CAS head field. Used only by enq.
     */
    private final boolean compareAndSetHead(Node update) {
        return unsafe.compareAndSwapObject(this, headOffset, null, update);
    }

    /**
     * CAS tail field. Used only by enq.
     */
    private final boolean compareAndSetTail(Node expect, Node update) {
        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
    }

    /**
     * CAS waitStatus field of a node.
     */
    private static final boolean compareAndSetWaitStatus(Node node, int expect, int update) {
        return unsafe.compareAndSwapInt(node, waitStatusOffset, expect, update);
    }

    /**
     * CAS next field of a node.
     */
    private static final boolean compareAndSetNext(Node node, Node expect, Node update) {
        return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
    }
}
